In rubber articles in which strength is particularly required, such as automobile tires, conveyor belts, hoses and the like, for the purpose of reinforcing a rubber and improving the strength and durability, used are steel cord-rubber composites, which are obtained by coating a metal reinforcing material, such as steel cord and the like, with a coating rubber. Here, in order to exhibit high reinforcing effect and obtain reliability in such a steel cord-rubber composite, a stable and strong adhesion is required between the coating rubber and the metal reinforcing material.
In order to obtain a steel cord-rubber composite exhibiting such high adhesion between the coating rubber and the metal reinforcing material, widely used is a method of embedding in a coating rubber compounded with sulfur a metal reinforcing material such as steel cords and the like plated with zinc, brass, etc., and simultaneously adhering the same with rubber vulcanization during heating vulcanization, i.e., direct vulcanization adhesion. Previously, in order to further improve the adhesion due to the direct vulcanization adhesion between the coating rubber and the metal reinforcing material, various studies have been performed regarding the direct vulcanization adhesion.
For example, PTL1 (JP2009-91691A) suggests a steel wire, in which a circumference of the steel wire is brass-plated, subjected to drawing process, and then a surface of the steel wire is washed in an aqueous solution containing a transition metal as a salt, and thereby the concentration of transition metals except for zinc and copper on the surface of the brass plating is set to 0.01 mass % or more.